Coastal zones are often described as effective filters for land-derived nutrient loads, referred to as the coastal filter, which applies in particular to bays and estuaries with higher water residence times. Open coastal zones are high energy environments in which the sediments are influenced by waves and currents, and residence time is short. Most open coastal zones are covered by permeable sandy sediments. Contrary to previous assumptions and despite their low organic matter content, permeable sandy sediments contribute to benthic nitrogen cycling and oxygen dynamics. However, there is a lack of understanding of how seasonality and physical processes govern denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in permeable sediments in shallow coastal zones. For this study, monthly field and laboratory experiments were conducted in an annual cycle, combining physical and biogeochemical measurements. Rates of denitrification and DNRA were measured with an adapted revised isotope pairing technique to simulate advective pore water flow during the incubations. Denitrification followed a seasonal cycle with higher N2-production rates observed in autumn and winter compared to summer. The data suggest that the oxygen penetration depth, activity of benthic primary producers and oxygen respiration are more dependent on prevailing ambient conditions, e.g. wave action, rather than on the season. Our results highlight the largely unknown nitrogen removal potential of permeable sediments. They also demonstrate the importance of including currents, waves, winds and past storms in the analysis of the biogeochemistry in permeable sediments in order to obtain a realistic picture of the prevailing processes.
How to cite: Gentsch, K., Ahmerkamp, S., Holtermann, P., Marzocchi, U., Thiele, O., and Voss, M.: The coastal nitrogen filter in highly dynamic permeable sediments of the southern Baltic Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11176, https://doi.org/10.5194/egusphere-egu25-11176, 2025.
Intertidal flats, as part of coastal wetlands, play a crucial role in the global carbon cycle as they bury substantial quantities of carbon belowground (126 Tg C y-1). Identifying the extent and rate of carbon turnover and thus greenhouse gas release from these ecosystems is essential to better understand their role in the global carbon cycle. Located at the interface of marine and terrestrial environments, intertidal flats may act as mixing zones between riverine nutrient input and incoming tides that carry nutrients and particles.
Eutrophication, i.e., the stimulation of phytoplankton primary production, is particularly concerning in coastal waters. Phytoplankton exude carbon; during its decomposition, further nutrients are released. The effect of such an input on intertidal flat carbon cycles is unknown. Here, we investigated greenhouse gas fluxes from an intertidal flat during a simulated algal bloom. We chose the Wadden Sea coast, northern Germany as a representative field site.
Preliminary characterization data suggested that CO2 fluxes at our field site are not limited by the presence of electron acceptors, e.g., sulfate, belowground but by the concentration and composition of organic carbon. Hence, we hypothesize that with the addition of organic carbon inputs during an algal bloom, higher CO2 fluxes are emitted. We performed tidally influenced microcosm experiments in which the sediment was inundated twice a day during high tide with algae enhanced artificial seawater. In addition, we also added organic carbon only (lactate/acetate) to determine if the co-occurring nutrients during the algal bloom, e.g., nitrogen play a role.
We observed that the algae treatment emitted twice as much CO2 compared to the control over the course of the experiment. In the recovery phase (after the algae bloom) the CO2 fluxes were still elevated compared to the control. The carbon only treatment showed CO2 fluxes 1.6 times higher than the control. Thus, not only carbon but also other nutrients that are part of the algal bloom control the CO2 fluxes from this ecosystem; for example, ammonium as a nitrogen source may enhance the release of CO2. Aqueous and solid Fe data show an increase in Fe(II) in the algae and carbon only treatment, indicating Fe(III) reduction. Both treatments also show increased acid volatile sulfide concentrations, suggesting sulfate reduction. The results show that CO2 release from the Wadden Sea increases significantly during an algal bloom and remains elevated during recovery. Therefore, further control on nutrient inputs to the Wadden Sea is necessary to prevent eutrophication.
How to cite: Kainz, N., Neumeier, C., Kappler, A., and Joshi, P.: Eutrophication events cause an increase in greenhouse gas release from intertidal flats, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2752, https://doi.org/10.5194/egusphere-egu25-2752, 2025.
Coastal intertidal wetlands are widespread along the French Atlantic coast. As blue carbon ecosystems, they can capture and sequester carbon in a sustainable manner. Local carbon density, net CO2 sequestration, CH4 emission, alkalinity production and its transfer to the ocean are regulated by a variety of factors, including salinity, nutrient loads, tides and seasonality. This study focuses on carbon sequestration as alkalinity in the intertidal zone of the Arcachon lagoon. Carbon dioxide capture through alkalinity production represents durable (>1,000 years) CO2 removal. Specifically, we studied the hydrology and chemical composition of waters draining from salt marshes at low tide by sampling these waters along a tidal creek, which is mainly fed by seepage of pore water from the salt marsh sediments. We also measured discharge rates. These measurements were taken in different seasons and at different tidal amplitudes. Our results show that brackish waters are systematically enriched in alkalinity and depleted in sulfate compared to the mixture between local seawater and freshwater endmembers. This suggests that sulfate reduction in sediments and sulfide precipitation in sediments are important processes in alkalinity generation. Measured fluxes show that for a drained salt marsh area of 30 ha, about 2,000 moles of C are sequestered as alkalinity (560 kgC/ha/yr), and transferred to the open Arcachon lagoon per tide. This indicates that salt marshes are efficient at sequestering carbon, not only as "blue carbon" but also as alkalinity. Therefore, tidal pumping is an important and generally overlooked process in this sequestration.
How to cite: Kanfer, P.: Expanding Blue Carbon horizons: Alkalinity as a key carbon sequestration pathway in Arcachon salt marshes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15217, https://doi.org/10.5194/egusphere-egu25-15217, 2025.
Due to their semi-closed hydrological nature, lagoonal ecosystems are the location of important biogeochemical transformations. Over the past century, these systems have accumulated organic matter (OM), pollutants, and nutrients, that was predominantly stored in sediments. Today, sediments are thus largely involved in elements cycles in shallow coastal areas through their role in oxygen consumption, OM and nutrient recycling and chemical contaminant release, and are further involved in the ecological degradation. Thus, sediment reactions and associated benthic fluxes of oxygen, nutrients and chemicals elements are essential processes that act as a critical driver of the biogeochemical coastal cycles and important to better understand the environmental and chemical degradation. This recycling may also limit the depuration role that sediment may play on carbon, nutrients and other pollutants storage by burial. Quantifying sedimentary contributions to ecosystem degradation and epuration is challenging. Indeed, mass balance of oxygen, nutrient, and pollutant exchanges at the sediment-water interface (SWI) has strong temporal variability in relation to temperature, quality of the OM and availability of oxidants that impact the efficiency of OM mineralization and burial and nutrients recycling over time. To address these gaps, the Bottom RedOx Model (BROM), a 1D diagenetic coupled benthic-pelagic modelling tool with O-N-P-Si-C-Fe-Mn-S biogeochemical module is particularly relevant as it is able to simulate organic matter production and mineralization, as well as major elemental cycles and trace elements fluxes at the sediment-water interface. It was applied in the Bolmon lagoon, a shallow Mediterranean lagoon impacted by eutrophication, deoxygenation and chemical pollution. In this study, the model was firstly calibrated using vertical profiles of diagenetic variables (O2, nutrients, trace elements, etc.) collected during seasonal field campaigns. It was then employed to better understand the diagenetic response to change in the physicochemistry of the water column and to reconstruct continuous fluxes over time and finally to estimate the net mass budget at the sediment-water interface. Results revealed substantial temporal variability in fluxes, predominantly driven by water column oxygen concentration and benthic macrofaunal activity that evolved seasonally in response to hydroclimatic and ecological conditions. The net mass balance highlighted that the sediment acted as a significant oxygen sink and nutrient source. Resulting OM degradation and burial was then discussed with respect to the prevalent physicochemical conditions in the lagoon. Comparison with riverine inputs underscored the sediment compartment as a critical factor influencing the ecological state of the Bolmon lagoon, necessitating its integration into future management strategies.
How to cite: Huchet, L., Berezina, A., Yakushev, E., Techer, I., and Rigaud, S.: Using BROM (Bottom RedOx Model) model to quantify the contribution of sediments in the eutrophication of a shallow Mediterranean coastal lagoon , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10234, https://doi.org/10.5194/egusphere-egu25-10234, 2025.
The Marano and Grado Lagoon, situated in the northeastern coast of Italy, is a shallow coastal ecosystem of significant ecological value at both regional and international levels. The effects of human activities related to local socio-economic development, alongside climate variability, necessitate a deeper understanding of the lagoon's trophic dynamics and the development of effective management strategies. In this study, we present a multi-year analysis of the biogeochemistry of the Marano and Grado Lagoon, utilizing both observational data and a high-resolution coupled physical-biogeochemical model that incorporates explicit benthic-pelagic interactions. This model integrates the transport model SHYFEM (available at https://github.com/SHYFEM-model/shyfem) with the biogeochemical model BFM (Biogeochemical Flux Model, bfm-community.eu/). The biogeochemical component simulates processes in both the water column and sediments, also ensuring their coupling. We conducted ten-year simulations for the physical-chemical properties (2006-2015) and five-year simulations for the biogeochemical processes (2010-2014). Our findings reveal that the distributions of the pelagic biogeochemical state variables align reasonably well with the observed data, while the concentrations of benthic state variables are consistent with established characteristics of the lagoon benthic environment.
How to cite: Scroccaro, I., Laurent, C., Aveytua Alcarez, L., and Canu, D.: Biogeochemical modeling of the Marano and Grado lagoon (Italy) adding a benthic-pelagic coupling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18936, https://doi.org/10.5194/egusphere-egu25-18936, 2025.
Dissolved CO2 and buried organic matter budgets have been studied in shelf seas to identify carbon fractions that are exported to the ocean interior or preserved in the sediment. However, the fate of suspended particulate organic matter remains less understood, particularly because its lability is difficult to identify. Analysis of the different fractions of particulate organic matter in the North Sea could contribute to understanding its fate. The particulate organic carbon (POC) concentration follows coastal-offshore gradients that can be predicted with the suspended particulate matter (SPM) concentration. The POC:SPM ratio indeed features a typical exponential decrease with the SPM concentration. While that ratio is higher offshore where SPM concentrations are minimum, it reaches low asymptotic values at the coast where SPM concentrations are high. Such a relationship is actually found in many different systems (coastal zones, estuaries) and at different latitudes. A semi-empirical model has been proposed to fit the observed data of that relationship in the southern North Sea (German Bight: Schartau et al., 2019; Belgian zone: Fettweis et al., 2022). Based on the model assumptions, it is possible to separate two fractions of POC: the fresh fraction, that is assumed to accumulate during the bloom and to be degraded within the season, and a more refractory POC fraction. More detailed calculations allow this latter fraction to be divided into a slow POC, which includes the refractory detritus, and a mineral POC, that is the POC adsorbed on the surfaces of clay minerals. We assume that suspended mineral particles in the North Sea provide a total surface area saturated with adsorbed organic matter, also considering an underlying dynamic equilibrium between adsorption and desorption of organic matter. We then calculate the SPM budget in the North Sea from satellite remote sensing and vertical concentration profiles obtained from in situ observations. On this basis, we can use the semi-empirical model to establish a budget of the fresh, refractory detrital and mineral fractions of POC on the shelf.
How to cite: Desmit, X., Riethmüller, R., Silori, S., Schartau, M., Van der Zande, D., and Fettweis, M.: Budgeting the particulate organic matter from the suspended particulate matter in shelf seas, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19833, https://doi.org/10.5194/egusphere-egu25-19833, 2025.
Seawater turbidity influences primary production, nutrient exchanges and the carbon cycle in our oceans. As turbidity is likely to increase throughout the twenty-first century, due to factors including the increased occurrence of storm events and anthropogenic pressures, the potential impacts on ecosystems must be investigated. Greater understanding of the relationship between light availability and ecosystem health will aid in forecasting the future biogeochemical state of shelf seas under a changing climate.
Seven sites across the North West European Shelf (NWES) are investigated to generate a picture of the impacts of turbidity in different environmental settings. For example, study sites with differing salinity concentrations, stratification or proximity to land represent the varied conditions around the British Isles. Hindcast data is crucial for depicting seasonal and interannual patterns in seawater turbidity on the shelf since 1990. We show that changes to chlorophyll increase turbidity on the NWES. Furthermore, the total optical absorption coefficient of water indicates turbidity across the shelf. In-situ measurement and earth observation data will also be used to calibrate model parameters and verify model outputs.
The hydrodynamic-ecosystem model ERSEM is used to represent the 1D water column. As light availability within the water column is determined by organic and inorganic particles, the specific shortwave backscattering properties (m2/mg C) of particulate organic matter (POM) are altered to represent greater turbidity. The effects on biogeochemistry, carbon, phytoplankton and zooplankton are studied to assess the response of the ecosystem to higher turbidity in European shelf seas. Results suggest that increasing the backscattering of POM has different impacts depending on environmental conditions. Increasing backscattering in the North Sea reduced microphytoplankton. Conversely, increased backscattering in the North Atlantic increased microphytoplankton.
Here we present the results of the 1D ERSEM turbidity analysis and supporting hindcast observational conclusions. Findings from the 1D ERSEM analysis and hindcast data will be utilized to inform the 3D ERSEM modelling of suspended particulate matter across the NWES. Future work will incorporate these findings into generating two environmental scenarios (such as increased temperature along with increased storminess and higher riverine discharge) to forecast the potential future of seawater turbidity on the NWES.
How to cite: Morton, R., Blackford, J., Schuster, U., van Maanen, B., and Rigby, S.: Clearing the Water on Ecosystem-Level Effects of Seawater Turbidity on the North West European Shelf., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10267, https://doi.org/10.5194/egusphere-egu25-10267, 2025.
The marine biological carbon pump plays a crucial role in the sequestration of atmospheric carbon and, therefore, is paramount to global climate regulation. Most existing research on biological carbon sinks has focused on understanding the role of oceanic (off-shelf) species and processes where the carbon can quickly be removed from contact with the atmosphere. We know little about how species living on continental shelves contribute to and influence carbon sequestration due to complex biological and physical transport process dynamics. This knowledge gap is becoming an issue as decision-makers seek to consider the impacts of anthropogenic pressures (e.g., fishing) on the flow and storage of carbon across shelf ecosystems. Most commercial fishing takes place in shelf seas. Fishing could impact carbon sinks and sequestration by altering population biomasses, ecosystem dynamics, and trophic interactions. Here, we explore the potential contribution of a selected fish population in the Irish Sea to carbon sinks and the impacts of fishing on that. The Irish Sea ecosystem, situated between Ireland and the UK, is a shelf ecosystem encompassing important commercial species populations, including herring, the species of interest in this study. An Ecopath with Ecosim (EwE) model of the Irish Sea has been developed to reconstruct the region's food web. We combine the Irish Sea EwE ecosystem model outputs of biomass with faecal egestion and attenuation rates under alternate fishing scenarios to provide a novel quantitative assessment of the annual flux of carbon that sinks to the continental shelf seafloor to then, in part, becomes buried in the sediment. Under the baseline fishing scenario, the Irish Sea Atlantic herring faecal pellet flux contribution was 84% higher than the carcass flux deposited on the seafloor. This carbon flux substantially increased for the non-fishing scenario with bigger changes in carcass carbon flux. Pelagic fish contribute significantly to the carbon flux, particularly with faecal pellets. The overall carbon deposition from the fish community changed little between scenarios with and without fishing on Atlantic herring due to food-web balancing. Our results provide an early insight into the relationship between commercial species, fishing, and biological carbon sink for shelf ecosystems.
How to cite: Silvar, P., Cavan, E. L., Martin, A., Bentley, J., Hill, S. L., and Reid, D.: Quantifying the fish contribution to on-shelf carbon sink: a case study of the Irish Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16411, https://doi.org/10.5194/egusphere-egu25-16411, 2025.
Variations in the elemental ratios of carbon, nitrogen, and phosphorus in marine organic matter (OM) and their influence on carbon cycling remain uncertain in both open and coastal oceans. While observations consistently show carbon enrichment and phosphorus depletion relative to elemental Redfield ratios, many biogeochemical models assume fixed Redfield stoichiometry. As a result, they often underestimate biological carbon fixation, limiting their ability to accurately represent carbon fluxes. Here, we provide a comprehensive assessment of the effects of variable OM stoichiometry on carbon cycling in the Northwest European shelf seas using the coupled 3D physical-biogeochemical modeling system SCHISM-ECOSMO-CO2. For this, we integrate two pathways for variable OM stoichiometry into the ecosystem model component ECOSMO: a release of carbon-enriched dissolved OM under nutrient limitation and the preferential remineralization of organic nitrogen and phosphorus. We evaluate both their individual and combined effects compared to a reference configuration with fixed Redfield stoichiometry. The variable stoichiometry configurations result in a 10-33% increase in annual net CO₂-uptake. This additional uptake is driven by enhanced OM cycling, with greater surface net autotrophy and subsurface net heterotrophy. As a result, the seasonal biological drawdown of DIC increases, enhancing the biological contribution to pCO₂ changes and shifting the maximum CO₂-uptake from winter to spring and summer. These results underscore the crucial role of variable stoichiometry in accurately representing the shelf carbon pump mechanism in the Northwest European shelf seas, as it has a significant impact on the efficiency of carbon sequestration. They also highlight the need to incorporate variable OM stoichiometry into regional and global biogeochemical models for a more accurate representation of the marine carbon cycle.
How to cite: Demir, K. T., Mathis, M., Kossack, J., Liu, F., Daewel, U., Stegert, C., Thomas, H., and Schrum, C.: Variable organic matter stoichiometry enhances the biological drawdown of CO2 in the Northwest European shelf seas, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10541, https://doi.org/10.5194/egusphere-egu25-10541, 2025.
Mercury (Hg) contamination in the Amazon is a recurring topic in scientific literature, with high concentrations reported in various environmental compartments, including water in dissolved and particulate fractions, soils, sediments, and fish. Despite the substantial body of research on Hg contamination in the Amazon, studies focusing on the coastal and continental shelf zones remain scarce, and, to our knowledge, no study has specifically evaluated Hg contamination in the waters of these areas. The Amazon coastal region and its continental shelf exhibit complex hydrodynamics influenced by the Amazon River discharge, ocean currents, and extensive mangrove systems, all of which can significantly affect Hg dynamics and transport. This study aims to investigate Hg concentrations in water and sediments in the coastal and oceanic region of the Amazon, identifying spatial trends and the main geochemical processes influencing its transport. The research covered the Amazon River estuary (Amazon Transect), the northern Amazon River plume (North Plume), the Pará River estuary (Pará Transect), an extensive mangrove area (Mangrove Belt), and a region influenced by the North Brazil Current (NBC). Sampling occurred during the high-discharge period of the Amazon River. Results showed that total Hg concentrations in water varied across regions, with the highest levels recorded in the Pará Transect and the lowest in the North Plume. These concentrations were positively correlated with total organic carbon and suspended particulate matter, which were identified as important geochemical supports for Hg transport on the continental shelf. Similarly, Hg concentrations in sediments reflected the patterns observed in unfiltered water. The Mangrove Belt and North Plume were significant zones of Hg deposition, while estuarine areas, such as the Amazon and Pará transects, acted as primary sources of Hg to the marine environment. Isotopic and elemental analyses of organic matter, combined with the observed negative relationship between Hg concentrations and the salinity gradient, suggest that the primary sources of Hg on the continental shelf are Amazonian soils and the resuspension of sediments from the shelf itself. These findings highlight that deforestation in the Amazon, a widely debated issue, extends its impact beyond terrestrial ecosystems, influencing water quality in the tropical Atlantic Ocean and posing risks to coastal and marine biota.
How to cite: Silva do Nascimento, L., Abreu Pestana, I., Seidel, M., Marques da Silva Junior, J., Cherene Vaz de Oliveira, B., Ribeiro Gomes, P., Koschinsky, A., Dittmar, T., and de Rezende, C. E.: Mercury concentrations along the Amazon estuary and plume: Spatial trends and geochemical processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11252, https://doi.org/10.5194/egusphere-egu25-11252, 2025.
In marine sediments with high content of reactive iron oxides, nearly all hydrogen sulfide, which is produced by microbial sulfate reduction, is reoxidized to sulfide oxidation intermediates (sulfur, thiosulfate, and sulfite) and, eventually, to the terminal oxidation product, sulfate. Such sulfur cycling is called “cryptic” and is found in a wide variety of marine and limnic systems. Cryptic sulfur cycling may be fast but leaves scarce geochemical evidence. One of the marine systems, which is characterized by a cryptic sulfur cycle in the sediments is the Gulf of Aqaba, Red Sea. The gulf is strongly affected by high fluxes of aeolian dust deposition from the adjacent deserts, especially from Sahara, which is rich in reactive Fe(III) and Mn(IV) phases. Multiple lines of evidence, including presence of trace amounts of hydrogen sulfide, sulfide oxidation intermediates and pyrite in the sediments, isotopic composition of sulfate as well as direct measurements of microbial sulfate reduction rates prove the presence of cryptic sulfur cycling in the sediments of the Gulf of Aqaba. Quantification of the rates of microbial sulfate and iron reduction as well as of abiotic oxidation of hydrogen sulfide in the sediments allows us to provide for the first time quantitative constraints on the cryptic sulfur cycling in marine sediments.
How to cite: Kamyshny, A. and Grijalva Rodriguez, T. I.: Quantitative constraints on the cryptic sulfur cycling in marine sediments affected by high fluxes of reactive iron from terrigenous sources, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8, https://doi.org/10.5194/egusphere-egu25-8, 2025.
Among the most important environments of the coastal zone are the estuaries where fresh water from land mixes with the sea water creating one of the most biologically productive regions on the earth. A wide range of human activities affect the estuaries and endanger their biodiversity due to unchecked development in many coastal areas across the world. A large fluctuation is observed in the pH in different parts of the estuaries. In view of the ocean acidification, it is important to delineate the pH tolerance range of the organisms thriving in the estuaries. In this study, we assess the effect of pH on the distribution and diversity of benthic foraminifera in a tropical estuary. The abundance and shell composition of both the living and recently dead benthic foraminifera was studied in the surface sediments collected from coarse sand rich pockets as well as the fine-grained organic matter rich sediments on the margins of the Terekhol estuary in the Goa state of India. The salinity, pH of the ambient water was noted and the sediment characteristics were analysed. The salinity ranged from 0.04 to 22.43 from the upstream station to the river mouth whereas pH varied from 6.99 to 7.84. The highest salinity and pH were recorded at the river mouth stations. Total Suspended Material (TSM) decreased from river mouth to upstream, with the highest TSM at the river mouth station (9.5 mg/l). The highest sand (%) was in the river mouth region and a higher percentage of sand was invariably observed in the middle part of the estuary. The sand and silt dominated the sediments throughout the estuary and the clay content was <24 % on all the stations. As compared to the middle channel stations, the sediments collected from the adjacent banks had higher organic carbon (Corg) content throughout the estuary. The highest Corg (4.52%) was observed at the upstream station. Except two stations in the river mouth, all of the stations in the Terekhol estuary showed higher Corg/N ratio (>10) indicating terrestrial input. A relatively higher CaCO3 (%) was observed in the river mouth region as compared to the upstream region. Benthic foraminiferal abundance was much higher in fine grained and Corg rich sediments on the banks as compared to sandy sediments devoid of the organic matter in the middle of the estuary. A distinct transition from the abundance of calcareous benthic foraminifera to agglutinated benthic foraminifera was observed at nearly neutral (7.14) pH. Ammonia dominated the calcareous benthic foraminifera at low pH, suggesting its wide pH tolerance range. This lower pH tolerance range is much lower than previously reported. Only, agglutinated benthic foraminifera were found at station with pH lower than 7.14. We report a much lower pH tolerance range of calcareous benthic foraminifera.
How to cite: Deepak, R. P. and Saraswat, R.: Transition from calcareous to agglutinated shelled benthic foraminifera at near neutral pH in a tropical estuary, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-297, https://doi.org/10.5194/egusphere-egu25-297, 2025.
The Bay of Bengal hosts the fourth most intense oxygen minimum zone (OMZ) globally, despite low primary productivity. Yet its microbial community and biogeochemical roles remain underexplored. We examined prokaryotic diversity in the euphotic zone and OMZ, revealing significantly higher alpha diversity in the OMZ than surface waters. Community structures varied between coastal and open ocean regions and within the OMZ across oxygen gradients. Proteobacteria dominated bacterial communities, with Cyanobacteria and Actinobacteria prevalent in the euphotic zone. In contrast, Marinimicrobiota, Marine Group B, Crenarchaeota, and others dominated the OMZ. Key genera included Prochlorococcus and SAR11 in the euphotic zone and SUP05 and SAR324 in the OMZ. Functional predictions indicated the prominence of denitrifiers, anammox bacteria, and sulfur oxidizers in the OMZ. This study underscores the critical role of microbial diversity in nitrogen and sulfur cycling in the Bay of Bengal.
How to cite: Nazirahmed, S., Rahi, P., Saxena, H., Singh, A., and Panchal, R.: High prokaryotic diversity in the oxygen minimum zone of the Bay of Bengal: Implications for nutrient cycling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1096, https://doi.org/10.5194/egusphere-egu25-1096, 2025.
Marine ecosystems are experiencing uneven changes in response to the synergistic effects of climate change and anthropogenic pressures. Phytoplankton primary productivity, driven by photosynthesis, fuels marine ecosystems, providing the source material for trophic transfer and carbon export to the ocean interior. This study provides a large-scale assessment of net primary production (NPP) and its temporal variability across different latitudinal bands (40° N–45°S) in the Asia-Pacific region over 25 years (1998 –2022), with emphasis on bloom phenology. NPP is estimated through multi-model inter-fusion using satellite-based observations, which offers practical means for broad regional assessments given the limitations of direct measurement techniques. Our analysis reveals a decline in NPP of −3.2% per decade (equivalent to −15.31 mg C m−2 day−1 per decade, P < 0.01). NPP decreases with increasing latitude in both hemispheres, reaching minimum values at 20°–25° latitude that are 1.75-fold lower than equatorial values. The observed NPP trends are coupled with changes in phytoplankton phenology. Using three phenological threshold methods, phytoplankton bloom initiation has advanced by approximately −1.1 ± 10.4 days per decade, while termination occurred about −3.3 ± 10.5 days earlier per decade, resulting in shortened bloom durations of around −2.8 ±15.3 days per decade. The seasonal cycle demonstrated increased reproducibility over these temporal shifts, reflecting more stable annual variations. These changes in productivity and phenology patterns reflect the influence of climate change and anthropogenic pressures across the Asia-Pacific waters, with implications for ecosystem energy flow, carbon sequestration processes, and species composition.
How to cite: Xu, Y., Xu, T., Botha, H., and Doran, B.: Coupled Changes in Marine Primary Production and Phytoplankton Phenology in a Warming Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-653, https://doi.org/10.5194/egusphere-egu25-653, 2025.
The Benguela Upwelling System (BUS) is one of the most productive marine ecosystems globally, providing vital ecological services and economic values. Under global warming conditions, upwelling intensification is expected to enhance the vertical ascent of nutrient-rich deep waters, fueling primary production. However, this intensification may also increase offshore lateral advection, potentially counteracting coastal ecosystem productivity. Enhanced offshore transport could reduce phytoplankton availability in coastal zones, affecting zooplankton and higher trophic levels, and weaken microbial activity by altering organic carbon sinking to deeper layers. This study quantifies the extent of Total Organic Carbon (TOC) lateral transport under varying upwelling conditions and examines associated changes in organic carbon pathways within the marine food web.
We employ a coupled physical-biogeochemical modeling system based on the Nucleus for European Modeling of the Ocean (NEMO v4.2.2) and the Biogeochemical Flux Model (BFM v5.3). A regional model configuration, encompassing the larger BUS domain at a 1/16° horizontal resolution, was nested into a global ocean model at 1/4° resolution using a two-way nesting approach. The BFM explicitly resolves pelagic-benthic coupling through sediment remineralization and adopts an intermediate-complexity structure to describe lower trophic-level ecosystem dynamics. The coupled model simulation spans 1980–2020, driven by ERA5 atmospheric forcing and GLOFASv2.1 runoff data.
Our results reveal that offshore lateral TOC transport in the upper 200 m of the Benguela region has steadily increased over the past four decades, with rates surpassing 2 mg C·m⁻²·s⁻¹ per decade. This trend reflects intensified coastal upwelling dynamics, which was more significant in the northern subregion during the austral spring season. Additionally, the displacement of organic matter export in the epipelagic zone towards the offshore open ocean highlights shifts in organic carbon pathways. These changes have also impacted the standing stocks of living and non-living groups within the lower trophic ecosystem, influencing carbon sinking dynamics across different pelagic zones.
How to cite: Talaat Salama, A., Lovato, T., Butenschön, M., and Zavatarelli, M.: How Intensified Oceanic Transports Shift Organic Carbon Pathways in the Benguela Upwelling System, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12112, https://doi.org/10.5194/egusphere-egu25-12112, 2025.
Rivers play a crucial role in transporting carbon from terrestrial watersheds to oceans. Understanding the quantity and characteristics of riverine organic matter discharged into the ocean is essential for predicting changes in marine and global organic carbon cycles. Riverine organic matter, comprising both allochthonous and autochthonous fractions, is influenced by shifts in watershed sources driven by climate change, as well as socio-economic transformations that affect its production and characteristics. Plastics, a significant source of allochthonous organic carbon, could contribute substantially to rivers, and microplastics (MPs) generated from plastic degradation may alter carbon cycling within river systems through interactions with other organic materials. Despite their importance, MP exports from rivers to oceans remain poorly quantified and rarely measured in terms of carbon mass, with even less understanding of their interactions with other forms of riverine organic matter. To address this gap, we have investigated the five major rivers in South Korea, accounting for 90% of the freshwater discharge. Here, we present the preliminary results for three major rivers, representing Korean fluvial system connected to the Yellow Sea. Both particulate and dissolved organic matters were characterized in quantitative and qualitative terms by monthly sampling at each river-mouth station, including particulate organic carbon (POC), chlorophyll-a, transparent exopolymer particles (TEP), Coomassie stainable particles (CSP), and MPs for particulate forms and dissolved organic carbon (DOC) and dissolved organic matter (CDOM and FDOM) of dissolved forms. Considering the spatiotemporal variability of organic matters and MPs, river samples were collected three times a day at 2-3 hour intervals and in each sampling by compositing the samples taken from horizontally three cross-sectioned sites and vertically 3–5 water column layers per site. This study aims to quantify the monthly loads of total organic carbon (POC and DOC) entering the ocean from these rivers, assess the relationships between various forms of organic matter, and determine the relative contribution of MP-derived organic carbon to total organic carbon. Our results are expected to provide valuable insights into the ocean load and their inter-relationships of various organic matter forms originated from fluvial system, and their potential impact on the marine carbon cycle.
Acknowledgement: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2024-00356940).
How to cite: Je, C.-Y., Kim, S.-K., Kim, J.-S., Song, N.-S., and Kim, T.-H.: Monthly Year-Round Characteristics and Ocean Export of Riverine Organic Matter: Relationship with Microplastics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9033, https://doi.org/10.5194/egusphere-egu25-9033, 2025.
Estuaries behave retention zones for anthropogenic pollutants, including urban waste and plastics. Under tidal influence, the river flow can be reversed depending on the strength of the tide (neap and spring conditions) and the freshwater discharge in the river basin. In the Guadalquivir River (south Spain), the basin is highly regulated, and most of its drainage area is constrain by a dam at the head of the estuary (ca. 100 km from its mouth). Here, we presents the results of a microplastic monitoring program in the Guadalquivir Estuary. Microplastic samples in sub-superficial waters (triplicates) were collected by net (200 microns mesh) on a bi-monthly basis (neap and spring conditions) in the period 2020-2022 (120 samples), under a wide range of environmental conditions. Most of the samples coincided with very low precipitation and freshwater discharge periods. Microplastics concentrations varied from 0 to 7 items per cubic meter, mostly fragments and films, and comprised a majority of polyethene and polypropylene particles (75%). Microplastics variability was not correlated with river current speed, salinity and turbidity, indicating the complexity of processes involved in the dynamics of microplastics under low freshwater discharge, when the tide dominates the river flow. These results can be considered as a current baseline, where concentration of microplastics is under 1 item per cubic meter (Quartile 3) for extended periods, while pollution peaks during flood periods in the estuary can exceed 2 orders of magnitude such baseline level.
How to cite: González-Fernández, D., Manzano, S., Quintana, R., Martín-García, A. P., Sirviente, S., and Sánchez-Guerrero-Hernández, M. J.: Microplastics dynamics under low freshwater discharge conditions in the Guadalquivir estuary, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19531, https://doi.org/10.5194/egusphere-egu25-19531, 2025.
Posters on site: Tue, 29 Apr, 14:00–15:45 | Hall X1
The Arctic Ocean and its coastal areas are especially vulnerable to climate change. Its ecosystem is rapidly changing in response to temperature increase, loss of sea ice, and freshwater input. However, the scientific community currently lacks sufficient information on the mechanisms and drivers behind the biogeochemical cycling of these additional inputs and the consequences that may arise for the Arctic environment.
This case study of Kongsfjorden, western coastal Svalbard, provides insights on how freshwater runoff from marine- and land-terminating glaciers influences the biogeochemical cycles and distribution patterns of carbon, nutrients and trace elements in an Arctic fjord system. We collected samples from the water column at stations along the fjord axis and proglacial river catchments and analyzed concentrations of dissolved trace elements (dAl, dV, dFe, dMn, dCo, dNi, dCu, dZn, dCd, and dPb), dissolved nutrients (nitrate, nitrite, silicate, phosphate), as well as alkalinity and dissolved inorganic carbon. Statistical tools were applied to identify and quantify biogeochemical processes within the fjord that govern the distribution of dissolved constituents. We found the biogeochemical cycles of Kongsfjorden to be influenced by the different chemical composition of proglacial and subglacial discharge but also by physically driven effects triggered by the glacier systems.
Our results suggest that the glacier type affects nutrient availability and therefore primary production. The subglacial discharge at the base of the marine-terminating glacier creates a highly turbulent zone in the inner part of the fjord, which transports nutrients from deep water to the photic zone. We found lower nutrient availability in areas of land-terminating glaciers due to less turbulent mixing and a more stratified water column. Consequently, this may lead to lower primary production compared to areas directly affected by marine-terminating glaciers.
Glacial discharge of both marine-terminating glaciers and riverine discharge of land-terminating glaciers are important sources for dissolved trace elements (dAl, dMn, dCo, dNi, dCu and dPb) that are involved in biological and scavenging processes within marine systems. The different weathering regimes of marine- and land-terminating glaciers result in different chemical signatures in proglacial and subglacial discharge. Our data shows that intensive carbonate weathering in proglacial catchments supplies fjord waters with additional dissolved carbonates and therefore attenuates reduced buffering capacities by glacial runoff.
We identified benthic fluxes across the sediment-water interface to supply fjord waters with silicate, dFe, dCu and dZn. We hypothesize these benthic fluxes are higher close to land-terminating glaciers, since more reactive particulate trace element species are generated by proglacial and riverine processes. This might drive benthic cycling and could lead to increased remobilization from the sediment.
This published study provides valuable insight into biogeochemical processes and carbon cycling within a climate sensitive high-latitude fjord region, which may help predict Arctic ecosystem change. As a result of the progressive glacier retreat, we predict Arctic fjords to become less productive ecosystems in the future. Ultimately this has the potential to alter the circulation of water masses and consequently change the redistribution of nutrients and essential trace elements in the water column. (https://doi.org/10.1029/2023GB008087)
How to cite: Thomas, H., Schmidt, C. E., Pröfrock, D., Steinhöfel, G., Stichel, T., Mears, C., and Wehrmann, L.: The Contrasting Role of Marine- and Land-terminating Glaciers on Biogeochemical Cycles in Kongsfjorden, Svalbard, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16690, https://doi.org/10.5194/egusphere-egu25-16690, 2025.
Thorium-234 (234Th; half-life = 24.1 days) has been used as an excellent tracer for estimating particle fluxes, including particulate organic carbon (POC) and trace elements, in the upper ocean. In this study, the distributions of total and particulate phases of 234Th (234Thtand 234Thp), POC, and other oceanographic parameters (temperature, salinity, and chlorophyll-a) were investigated in the North Equatorial Currents (NEC) during September 21-27, 2024. The study region, located within ; 13.5˚N and 134-157˚E, is interconnected with the Kuroshio Current and Mindanao Current. Meausrments were conducted in the upper 500 m of the water column. The calcualted Th fluxes at a depth of 100 m ranged from 1730 to 2850 dpm m–2 d–1, comparable to fluxes observed in other open-ocean regions. A subsurface chlorophyll maximum (SCM) layer was observed at depths of 125–150 m, indicating oligotrophic conditions in the study area. Notably, the activity of 234Thp was elevated within the SCM layer compared to other depths. These finding highlight the importance of understanding biogeochemical cycles in the NEC, which plays a crucial role in regional and global carbon and nutrient cycling.
How to cite: Seo, J., Oh, C., and Kim, I.: Preliminary results: Distributions of POC fluxes in the North Equatorial Currents of the Pacific Ocean , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3437, https://doi.org/10.5194/egusphere-egu25-3437, 2025.
Ocean carbon uptake is essential to the global carbon cycle, as the ocean absorbs about one third of the atmospheric carbon released by human activities. Central to this process is the biological carbon pump, fueled by phytoplankton primary production. This pump transports organic carbon from surface waters to the ocean's deeper layers, where it can remain sequestered for hundreds to thousands of years. Coastal shelf regions, with their shallow, nutrient-rich waters, play a particularly active role in this cycle, supporting higher productivity levels compared to the deeper open ocean. However, in these shallow waters, organic carbon is more likely to be remineralized before it reaches the deep ocean. In these coastal zones, submesoscale filaments—or streamers—form through the instability of coastal currents, creating long, narrow structures that concentrate phytoplankton and chlorophyll. These streamers enhance long-distance transport of organic material, carrying chlorophyll-rich waters from the productive shelf regions into the open ocean, potentially increasing carbon flux to deeper ocean layers. A systematic estimation of how much carbon is transported by these structures is missing from the literature, due to the challenges in detecting and measuring streamers, which exhibit strong time variability over scales of several days. To address this gap, our work proposes a K-means based framework to detect streamers from chlorophyll and sea surface temperature satellite data in the Pacific and Atlantic Eastern Boundary Upwelling regions, and estimate their associated lateral carbon transport from satellite ocean color data. By estimating this cross-shelf export, we aim to deduce the potential sequestration rate, assuming that offshore high-chlorophyll streamers might increase the sinking fluxes of organic carbon to the deep ocean. This method relies solely on satellite products and can be operationalized for constant monitoring of the process in the years to come.
How to cite: Benassi, F., Pinardi, N., Mentaschi, L., Federico, I., Bianconcini, S., and Siedlecki, S.: Shelf water streamers: a potential pathway for ocean carbon sequestration in the Atlantic and Pacific Eastern Boundary Upwelling Systems, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5041, https://doi.org/10.5194/egusphere-egu25-5041, 2025.
Volcanic tephra produced from explosive volcanism modifies chemical cycles in local and regional environments. In tropical volcanic island settings, pristine tephra produced from explosive volcanism is deposited directly into the surrounding seawater and impacts the physicochemical conditions of the marine ecosystem. Additionly, intense weathering of terrestrial deposits prolongs the input of tephra into the coastal environments for extended periods. Coral reefs growing around the coasts of volcanic islands react to changes in surrounding seawater conditions which is the basis of many tropical palaeoclimate studies. We use a common massive Caribbean coral Diploria strigosa to investigate volcanic distrubances in the physicochemical conditions of the marine ecoystem following the April 2021 explosive eruption of La Sourfrière (St. Vincent). A coral core was sampled from the north-west coast of Barbados in July 2022 and from St. Vincent in March 2024. These locations were chosen to compare how tephra quantities and grainsizes differences influence the physicochemical conditions and consequenlty the geochemical imprint of the eruption in coral skeletons. Element/Ca ratios were measured along the theca of each sample using LA-ICP-MS analyses. Our preliminary results from the proximal sample show clear disturbances in the seasonality of common geochemical proxies used for sea surface temperature calibrations (i.e. Li/Mg, Sr/Ca, U/Ca). Additionally, we show that Pb/Ca, Y/Ca, La/Ca and Nd/Ca are elevated following the eruption and exhibit seasonality which was absent prior to the eruption. The seasonal signal decreased progressively each year following the eruption and it interpreted to represent seasonal runoff during the rainy season.
How to cite: Vincent, J., Flöter, S., Tsay, A., and Shelkdrake, T.: LA-ICP-MS analyses of Caribbean coral Diploria strigosa reveals deviations in geochemical trends following explosive eruption of La Soufrière in April 2021., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9692, https://doi.org/10.5194/egusphere-egu25-9692, 2025.
Nitrogen fixation by cyanobacteria in the Baltic Sea plays a crucial role in the context of eutrophication, as it promotes biomass production in the absence of dissolved inorganic nitrogen (DIN). Its contribution to the N budget is comparable to the combined sum of riverine and airborne DIN input, ranging from 300 kt-N/yr to 800 kt-N/yr. The vast range is due to internal fluctuations and significant uncertainties in various techniques used to determine N2 fixation and in extrapolate local studies to entire basins. To overcome some of the limitations we introduce a new approach based on large-scale records of the surface water N2 depletion during summer.
For our studies we use a membrane contactor (Liquicel) to establish gas phase equilibrium for atmospheric gases dissolved in seawater. The mole fractions of N2, Ar and O2 in the gas phase are continuously determined by mass spectrometry, yielding the concentration of these gases by multiplication with the total pressure and the respective solubility constants.
Thorough laboratory tests demonstrated that our Gas Equilibrium–Membrane-Inlet Mass Spectrometer (GE-MIMS) has sufficient accuracy and precision to detect and quantify nitrogen fixation. In June/July 2023, the GE-MIMS was deployed (i) on a voluntary observing ship (VOS, “Finnmaid”) for surface water gas analyses and (ii) for vertical water column studies on RV Elisabeth Mann Borgese along the VOS route between Helsinki and Travemünde. The VOS campaign enabled repeated transects along the same route and providing high spatial and temporal resolution time series of N2 concentration changes due to nitrogen fixation. First results clearly indicate regions and episodes where N2 fixation was active. Concurrent records of pCO2 obtained from a different measurement system, along with O2 concentrations, will be used for an independent characterization of cyanobacterial biomass production and thus of the associated N2 fixation. Additionally, Ar measurements are used to account for the air-sea gas exchange.
Our objectives are to identify factors initiating and limiting cyanobacteria growth, with the final goal of determining Baltic Proper's N2 fixation capacity.
How to cite: Iwe, S., Schmale, O., and Schneider, B.: Development of a Gas-Equilibrium Membrane-Inlet Mass spectrometer (GE-MIMS) for continuous N2, Ar and O2 measurements to quantify nitrogen fixation in the Baltic Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10437, https://doi.org/10.5194/egusphere-egu25-10437, 2025.
Understanding the transport of microplastics in coastal and estuarine areas is critical for assessing their global distribution and ecological impact. These regions act as dynamic interfaces where microplastics can accumulate, transform, and be transported to the open ocean or coastal sediments. In this study, we investigate the seasonal transport dynamics of microplastics in Arcachon Bay (SW France), a tide-dominated coastal lagoon characterized by 70% intertidal flats, using a numerical Lagrangian model combined with in-situ observations. Modeled trajectories were validated against Lagrangian drifter data under various conditions, showing remarkable agreement, including beaching and refloating processes at sandbars during ebb and flood tides, respectively. In-situ observations of microplastic concentrations and properties, collected in April 2019 from the sea surface, water column, and intertidal bottom sediments at single times across five stations, were used to set up the numerical model and to contextualize and discuss the simulation results. Trajectories of the three prevalent particle categories (low-density fragments, PET fibers, and rubber fragments) were simulated from major sources (rivers, sewage, port, and fishing areas) across two contrasting seasons, with trends compared to in-situ observations. The analysis is expected to reveal distinct seasonal transport pathways influenced by particle properties and hydrodynamic conditions, providing insights into dispersal patterns, retention zones, and potential hotspots for microplastic accumulation. The modeling results aim also elucidate transport patterns suggested by localized observations, such as the presence of hotspots of low-density particles at the sea surface within the channels, the greater abundance of fibers and rubbery particles outside the bay, the role of intertidal channels in flushing or retaining different particle types, and the combined influence of source location (e.g., sewage) and hydrodynamics on these distributions.
How to cite: Le Pedevic, A., Jalón-Rojas, I., Lefebvre, C., Cachot, J., and Morin, B.: Microplastic Transport Trends in a Tide-Dominated Coastal Lagoon: Insights from Arcachon Bay (SW France), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10695, https://doi.org/10.5194/egusphere-egu25-10695, 2025.
In spite of considerable efforts using approaches combining observations and modeling, the mechanisms governing the exchange of carbon dioxide (CO2) at the air-sea interface and the spatio-temporal variability of this exchange in coastal oceans are not yet fully understood. The present study uses simulations performed over the last decades with the global ocean biogeochemical model MOM6-COBALT to quantify the relative contributions of thermal changes, oceanic transport, freshwater fluxes, and biological processes to the spatial and seasonal variability of CO2 sources and sinks in the coastal ocean worldwide. These results allow identifying five distinct coastal domains each characterized by different behaviors: coastal regions dominated by biological carbon drawdown, regions controlled by vertical transport, influenced by land-derived inputs, regions shaped by intracoastal alongshore currents, and regions with weak CO2 fluxes. Using the spatial distribution of these behaviors, we propose a new, process-based delineation of the global coastal ocean that reflects the dominance of specific controlling processes for the spatial and seasonal dynamics of the CO2 exchange at the air-sea interface.
Our results also reveal that the spatiotemporal variability of CO2 fluxes in coastal regions is primarily driven by exchanges with the open ocean and local intra-coastal processes, while the influence of continental inputs remains confined to specific hotspot areas. In addition, although thermal changes are often associated with seasonal CO2 variability, their dominance stems from compensating effects between larger non-thermal processes, particularly biological drawdown and vertical transport.
The classification of the global coastal ocean presented in our study provides an updated process-based vision of the complex interplay between physical and biogeochemical drivers of CO2 exchange at the air-water interface. These findings provide a more comprehensive framework for understanding coastal CO2 dynamics and their role in the global carbon cycle, offering valuable insights for predicting the responses of coastal regions to both natural and anthropogenic environmental changes.
How to cite: Regnier, P., Roobaert, A., Laruelle, G., Liao, E., and Resplandy, L.: Unraveling the physical and biological controls of the global coastal CO2 sink, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12048, https://doi.org/10.5194/egusphere-egu25-12048, 2025.
Coastal lagoons are ecologically and socio-economically important because they provide valuable services. In Italy, the lagoon of Goro (Ferrara) is characterised by numerous shellfish farms, where the most cultivated species are clams. However, these environments are subject to various pressures, both natural, such as variations in temperature, nutrients and salinity, and artificial, such as the pollution caused by microplastics. Contamination of the sediments and thus of the animals reared on the sandy seabed is possible. As microplastics are an emerging contaminant and there is no limit to the amount of microplastics found in livestock, it is important to track the movement of livestock from seed to sale. Software is being developed to digitise all the steps along the supply chain, allowing consumers to know the origin of the produce, whose quality is judged by the amount of microplastics found in the sediments and in the animals. The result will be an assessment not only of the quality of the product, but also of the level of pollution in the lagoon where the clams have been farmed.
How to cite: Pignoni, E., Fini, C., Bettini, G., Botezatu, C., Sfriso, A., Corbau, C., Giannelli, C., Stefanelli, C., and Coltorti, M.: Traceability as a tool for identification of microplastic pollution and healthy food in the Goro lagoon , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15582, https://doi.org/10.5194/egusphere-egu25-15582, 2025.
Coastal sediments are known for their role in long-term carbon sequestration. Bathymetric depressions in coastal settings accumulate fine grained organic-rich material, removing carbon from biogeochemical cycles in the water column. However, the importance of shallow near-shore settings as carbon sinks is yet to be explored. Sediments in such settings may contain large amounts of carbon from various marine and terrestrial sources, that may be either buried in-situ or transported to deeper areas. Transport and deposition of organic matter in the littoral zones of the Baltic Sea are highly complex. In this study, hand core sediments and porewaters were analysed to quantify the spatial distribution of dissolved and particulate carbon over a strong gradient of salinity and wave exposure in shallow locations (3–4 m water depth). We compared four groups of locations in the coastal zone: sheltered estuary, sheltered archipelago, semi-sheltered archipelago, exposed shoreline, in each case determined sediment carbon stock and microbial turnover as a fraction of the stock. Results were also compared against data for deeper known sites of sediment accumulation in the Baltic Sea. In the shallow near-shore settings, highest carbon stocks (~4000g C/m2 in the uppermost 25 cm) were observed in sediments in the low wave energy systems (sheltered estuary and sheltered archipelago). Grain size analysis confirms that these areas are characterized by relatively fine material (d50 is <63μm). In contrast, exposed shoreline areas were generally sandy and had carbon stocks one to two orders of magnitude lower than in the sheltered locations. Using porewater profiles to estimate diffusive fluxes of dissolved inorganic carbon, methane, and dissolved organic carbon, we observed variable rates of carbon turnover between the sampling locations, expressed as a fraction of the stock remineralised in a one-year period. Our findings highlight the importance of sheltered near-shore sediments in the carbon budget and dynamics of this coastal system and underscore the need to capture small-scale spatial heterogeneities when quantifying the fate of organic carbon in coastal settings.
Keywords: Carbon sequestration, Coastal sediments, Sediment dynamics, Baltic Sea
How to cite: Nishant, N., Cowie, G., and Jilbert, T.: Assessing Sediment Carbon Stocks and Microbial Turnover in Shallow Coastal Areas of the Baltic Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15890, https://doi.org/10.5194/egusphere-egu25-15890, 2025.
Marine gels are organic polymers spanning from dissolved Exopolymeric Substances (EPS; nm to µm) to Transparent Exopolymer Particles (TEP; µm to mm), playing a key role in marine ecosystems by enhancing flocculation between organic and mineral particles. This process significantly affects the size distribution, density, and vertical transport of Suspended Particulate Matter (SPM) as well as the carbon cycle in the ocean. In turbid coastal environments, TEP produced by phytoplankton determines the seasonality of SPM concentration and influences the export of particulate organic matter. Although the biogeochemical importance of TEP and EPS is now recognized, the factors controlling their production by phytoplankton remain poorly understood and their dynamics is seldom included in biogeochemical models.
This study combines experimental laboratory approaches with mechanistic numerical modeling to decipher the complex relationships between light intensity, interspecific variation, and marine gel production in a turbid coastal zone. Laboratory experiments were conducted on six representative marine diatom strains isolated from the coastal Belgian Part of the North Sea. Following the carbon overflow hypothesis, which suggests that excess cellular internal carbon compared to nutrients leads to EPS excretion and subsequent TEP formation, the strains were subjected to varying light intensities. EPS and TEP concentrations were measured along with phytoplankton and bacterial abundances, as well as particulate organic carbon and nitrogen concentrations during exponential and stationary growth phases. This set of experiments is used to further develop a zero-dimensional biogeochemical model initially designed to simulate dissolved organic matter production and TEP formation during a mesocosm diatom bloom.
Analysis of EPS production revealed distinct patterns across strains, with maximum specific EPS production rates varying by 35%. No correlation between mean cell volume and specific EPS or TEP production was found. Despite the absence of a systematic correlation, specific production rates and TEP formation were generally higher under high light conditions, supporting the carbon overflow hypothesis. The smallest taxon Skeletonema sp. exhibited irregular EPS dynamics with significant losses, suggesting interspecific differences in EPS reactivity and bacterial activity. Cellular C:N ratios remained stable (5-7 mol C:mol N) across all conditions, indicating maintained internal stoichiometry in stationary phase, with no clear relationship to EPS or TEP production.
Preliminary model simulations showed increased TEP:phytoplankton biomass ratios under high irradiance conditions compared to moderate and low light conditions, agreeing with the more similar and lower TEP production reached in medium and low light experiments. Yet, despite the low interspecific variation in maximum EPS production rates suggesting that a homogeneous parameterization could be used, other resource acquisition parameters are known to vary with cell size, and the current constant model parameterization could not allow the adequate simulation of all experiments. This set of experimental data and simulations shows that the underlying mechanisms controlling marine gel production require further investigation and improvement of our biogeochemical models to better represent particle and carbon dynamics in coastal systems.
How to cite: Jourdevant, Y., Brun, A., Amadei Martínez, L., Sabbe, K., Desmit, X., Laruelle, G. G., Capet, A., and Terseleer, N.: Marine gel production in coastal diatoms studied by experimental and modeling approaches under varying light conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19291, https://doi.org/10.5194/egusphere-egu25-19291, 2025.
Iron separation from different solid phases is essential for evaluating the environmental function of iron in sediments. Oxalate is a commonly used extractant that effectively extract iron(hydro)oxides in sediments by complexation. However, when using spectrophotometry method, excess oxalate will interfere the complexation of iron with 1,10-phenanthroline resulting in the failure of iron measurement. In this study, we discovered an effective method for spectrophotometric analysis of iron samples with oxalate by adjusting the pH to 7-9, which changes the structure of the Fe-oxalate complexes and ensured the complexation of iron with 1,10-phenanthroline. Further exploration indicates that photolysis and heating also decompose Fe-oxalate complexes, but the performance is not as good as pH adjustment. The standard solution exhibits a strong linear relationship between absorbance (Abs) and concentration (Con): Abs = 0.1934 × Con + 0.1360, with a R2 of 0.9997, accuracy of 97.1%, and precision of 98.6%, which demonstrate the reliability of this method. Overall, the pretreatment is simple, and the influence of the organic solvent (oxalate) is diminished after pH adjustment. This method is expected to contribute to community by providing a new reliable, effective, less pre-treatment, costive, and sensitive testing approach and hopefully assist in the investigation on environmental function of iron minerals.
How to cite: Zhang, Y. and Liu, Y.: A New 1,10-Phenanthroline Method for Oxalate-extractable Iron Measurement, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6800, https://doi.org/10.5194/egusphere-egu25-6800, 2025.
Posters virtual: Wed, 30 Apr, 14:00–15:45 | vPoster spot A
EGU25-19333 | ECS | Posters virtual | VPS4
Spring-neap tidal variation of fluid mud occurrence in the hyper-turbid Ems estuaryWed, 30 Apr, 14:00–15:45 (CEST) | vPA.22
Dredging of the fairway in the Ems estuary was driven by the need to accommodate the increasing draft of ships. This modification has had negative effects on the sediment balance and ecology of the estuary. The fairway deepening results in strong alterations of the tidal dynamics, such as tidal amplitude and duration, as well as hydrodynamics such as current velocity and turbulence. This results in increased fine sediment input, which, at high suspended sediment concentrations, contributes to the formation of fluid mud—a mixture of silt, clay, and organic matter. The dynamics of fluid mud, particularly the differences between spring and neap tides, are not yet fully understood.
We investigated the formation, dispersion, and entrainment of fluid mud during the semi-diurnal tidal cycle. Therefore, the influence of flow velocity and salinity at different water depths, and the differences between spring and neap tides based on two dedicated measurement campaigns in 2023 was analyzed using high-resolution spatiotemporal monitoring data. Salinity data were used as an indicator of stable stratification. Additionally, sediment samples have been collected using a sediment corer to analyze the composition and properties of the fluid mud layer.
Our Spring-neap tide analysis showed a reduction of the flow cross-section during neap tide leading to differences in hydrodynamics between spring and neap tide driven by high sediment concentrations and fluid mud occurrence. During neap tide fluid mud was found to cover a larger fraction of the water column than during spring tide. This further highlights the strong influence of flow velocity on the dynamics of fluid mud and the need to include spring-neap considerations for future sediment management plans for the Ems.
How to cite: Lehn, J., Slabon, A., Holthusen, D., Rovelli, L., Fiskal, A., Hoffmann, T., and Borgsmüller, C.: Spring-neap tidal variation of fluid mud occurrence in the hyper-turbid Ems estuary , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19333, https://doi.org/10.5194/egusphere-egu25-19333, 2025.
